Suzanne Vega, Sting and Alanis Morissette are among the artists appearing on the Songs for Tibet album.

Access to Apple's online iTunes Store has been blocked in China after it emerged that Olympic athletes have been downloading and possibly listening to a pro-Tibetan music album in a subtle act of protest against China's rule over the province.

The album, called Songs for Tibet, was produced by an a group called The Art of Peace Foundation, and features 20 tracks from well-known singers and songwriters including Sting, Moby, Suzanne Vega and Alanis Morissette.

It was released as a download on the iTunes Store on August 5 - three days before the start of the Olympics - with the physical CD launched on Tuesday this week.

The Foundation provided free downloads of the album to Olympic athletes, urging them to play the songs on their iPods during the Games as a show of support.

Funds raised from the sale of the album are being used by the non-profit Foundation to support "peace-related projects that are dear to the Dalai Lama", the exiled Tibetan spiritual leader whom China regards as subversive.

On Monday, expatriate iTunes users living in China began experiencing technical problems with their previously unfettered access.

That was the same day the US-based Campaign for Tibet organisation claimed on its website that "over 40 Olympic athletes in North America, Europe and even Beijing" had downloaded the album.

Although some iTunes account-holders suggest that the problem is with Apple, according to several forum posters and bloggers working in China, the source of the technical hitch is being attributed to the Great Firewall of China - the umbrella term given to China's system of internet censorship.

A blogger calling herself JeninShanghai has reposted what she says is a reply she received from Apple's customer support after reporting that she had problems with her US iTunes Store account.

"iTunes is not being blocked in China from our end, but access to the iTunes Store IS restricted in some areas in China. This would also explain why it's happening to your friends there as well," the response reads.

"I would advise that you contact your ISP [internet service provider] about this matter. Please also note though that accessing the US iTunes Store outside of the geographic region of the United States is not supported, and that attempting to access it while in China is at your own risk."

The apparent blocking of the iTunes Store raises some thorny issues for Apple which opened it's first bricks-and-mortar store in Beijing on the eve of the Olympic Games.

"This is the first of many stores we will open in China," said Ron Johnson, Apple's senior vice president of retail, in remarks at the store opening in July.

The company, whose CEO and co-founder Steve Jobs is a practising Buddhist, is also negotiating with Chinese mobile phone operators to launch the highly successful iPhone.

iTunes Stores are locked to specific countries. In the case of the US iTunes Store - the biggest music retailer in the country - only holders of US-issued credit cards or US-issued iTunes gift certificates can purchase music and videos.

Many US, European and Australian expatriates living in China access their home-based iTunes Store accounts from China where they can purchase and download music , videos and podcasts. However, because of the block, that is no longer possible. The album is currently available on the Australian iTunes Store.

According to a report published in the semi-official Chinese news portal, china.org.cn, "angry netizens [internet users] are rallying together to denounce Apple in offering Songs for Tibet for purchase. They have also expressed a wish to ban the album's singers and producers, most notably Sting, John Mayer and Dave Matthews, from entering China."

The report, published in English, goes on to say that some netizens are even calling for a boycott of Apple products, including the iPhone when it is eventually released in China.